A communication system is formed, at a minimum, of a transmitting station and a receiving station interconnected by a communication channel. Communication signals generated by the transmitting station are transmitted upon the communication channel to be received by the receiving station.
A radio communication system is a communication system in which at least a portion of the communication channel is formed of a portion of the electromagnetic spectrum. Increased mobility of communications is permitted as a fixed or hard-wired connection is not required to be formed between the transmitting and receiving stations.
A cellular communication system is an exemplary radio communication system. A subscriber to a cellular communication system, when positioned at almost any location throughout an area encompassed by the network infrastructure of the cellular communication system, is able to communicate by way of the system with a mobile terminal.
The network infrastructure of an exemplary cellular communication system includes spaced-apart, fixed-site base stations which include transceivers. In such an exemplary system, each fixed-site base station defines a cell. As the mobile terminal used by the subscriber to communicate with another communication station travels between cells of the system, uninterrupted communication is possible by handing-over communications from one base station to another.
Several analogous types of wireless communication systems have been implemented, and others have been proposed, to encompass limited areas, such as the area encompassed by a building structure or office workplace. Wireless communication systems sometimes referred to as microcellular networks, private networks, and WLANs (wireless local area networks) are exemplary of such systems.
Wireless communication systems are typically constructed pursuant to standards promulgated by a regulatory or quasi-regulatory body. For instance, the IEEE 802.11 standard, and variants thereof, promulgated by the IEEE (Institute of Electrical and Electronic Engineering) is a wireless LAN standard pertaining generally to communications at various wireless frequencies including a 5 GHz range and a 2.4 GHz range. The 802.11 standard specifies an over-the-air interface between a wireless client, e.g, a mobile terminal, and a base station or access point, as well as among wireless clients. Standards pertaining to a physical layer and an MAC (media access control) layer are set forth in such standard. The standard permits automatic medium sharing between different devices which includes compatible physical layers. Asynchronous data transfer is provided for in the standard, generally by way of the MAC layer which utilizes a CSMA/CA (carrier sense multiple access with collision avoidance) communication scheme.
While the IEEE 802.11a standard requires that data be able to be communicated at relatively high data rates, e.g., data rates in excess of 54 Mbits/sec, achieving such a throughput rate is difficult when the communication channel upon which the data is transmitted exhibits fading characteristics. The high data rate communication is required to be effectuable within a stringent spectral efficiency range, a stringent error rate performance level and within appropriate computational load requirements. At a data rate of 96 Mbits/sec, a spectral efficiency of six bits/s/Hz, including coding, is required. The 96 Mbits/sec data rate is desired to be effectuated with an error rate performance at least as good as the error rate performance set forth in the IEEE 802.11a standard at a reduced rate, e.g., a 54 Mbits/sec data rate.
Generally, to overcome distortion introduced upon a signal transmitted upon a channel which exhibits fading characteristics, various techniques are utilized.
Time encoding of the signal, prior to its transmission, is sometimes utilized to counteract the distortion introduced thereon during its transmission upon the channel. Time encoding introduces signal redundancy upon the signal. By increasing the time redundancy of the signal, the likelihood that the informational content of the signal can be recovered, once received at the receiving station, is increased. Introducing time redundancy into the signal is sometimes referred to as creating time diversity.
Space diversity is sometimes also utilized to overcome the distortion. Typically, space diversity refers to the utilization of more than one transmitting antenna transducer from which a communication signal is transmitted, thereby to provide spatial redundancy.
And, various modulation techniques are also utilized. TCM (Trellis-Coded Modulation), for instance, is sometimes utilized. Conventional TCM techniques typically utilize an acceptable number of states, e.g., less than 256 states. But, a resultant trellis that is utilized in conventional TCM requires parallel transitions between states. The performance of a TCM scheme with parallel transitions provides almost no coding gain, relative to an uncoded communication scheme, when operated in a fading environment.
Multiple TCM is sometimes also utilized. In a TCM scheme, multiple coded symbols are transmitted during each transition in the trellis defined in the modulation scheme. Coding gains, sometimes almost 3 dB in magnitude, are sometimes achievable in the use of multiple TCM in contrast to conventional TCM. But, each increase in the effective length of the multiple-TCM requires another coded symbol to be added per transition. Additionally, each additional coded symbol requires another set of encoders, thereby increasing the complexity of the code. Additionally, coded symbols of the M-TCM require orthogonal channels for transmission. As spatial dimensions are generally not orthogonal, orthogonal time slots or frequency carriers are required to be used. The use of orthogonal time slots or frequency carriers would be contrary to the requirement to maintain spectral efficiency.
Additionally, trellis STC is sometimes utilized. When trellis STC is utilized, at least as many states are required as there are transitions per state. In other words, a trellis STC implementation would generally require at least 256 states. Such a trellis utilizing two-dimensional constellation is unable to provide a coding gain, merely a diversity gain, and thereby also is inadequate. ED-TCM (Enhanced Dimensional Trellis-Coded Modulation) is also sometimes utilized. In ED-TCM, subsets are selected from a Cartesian product constellation. Through such selection, though, the required spectral efficiency cannot be achieved.
As a result, while various modulation schemes are used to combat the effects of fading exhibited upon a communication channel, the existing modulation techniques are not able to ensure that data can be communicated at high data rates as required, e.g., pursuant to the IEEE 802.11 standard, while also maintaining acceptable spectral efficiency levels, error rate performance, and appropriate computation complexity levels.
If a manner could be provided by which better to modulate data which is to be communicated at high data rates upon a channel susceptible to fading, improved communication quality levels when operating a communication system would be possible.
It is in light of this background information related to wireless communications that the significant improvements of the present invention have evolved.